At any rate, fun to hear more about your plans. I'm another member of the "removed roof Air-Con" club
I think if I were to make a "one time" type trip to Florida in July or Phoenix in August that I might either go with a portable Air-Con between the front seats (they are shaped sort of like one of those oil-filled radiator heaters), or perhaps buy a cheap 5,000 BTU window air conditioner, cut a plywood "mount" and put it in the driver or passenger door window (have seen someone do this in a Class B van). Then I'd just sell the unit on Craigslist or something upon departure. Typically, I tend to boondock, and my idea of misery would have been to run the generator AND the roof air-conditioner.
To each their own though, and preferences and situations vary widely. The "standard" 14" roof hole for vent or A/C makes it fairly easy to customize, with the option to change back.
I can't say as I've noticed any big jumps in gas mileage without the roof A/C, but I also don't keep that close a track, plus my driving situations aren't that consistent (towing/not-towing/mountains/flat/putzing along/going faster/etc.). I'm sure it can't have hurt though.
